1. Field of the Invention
This invention relates to a method of making a carrier tape for feeding lead wires to semiconductor integrated circuit (IC) devices.
2. Description of the Prior Art
Although connections between electrodes on IC devices and external leads are generally made by wire bonding, it has been proposed recently to use a carrier tape as shown in FIG. 1 in order to automate the packaging of IC devices.
This carrier tape 1 is made of a metallic foil tape of copper, iron, iron-nickel alloy, etc., having a thickness of about 200 microns or less, and includes sprocket holes 2 provided at intervals along both edges of the tape, clusters of finger leads 3 provided at intervals along the middle of the tape. The surface of the metallic foil tape is entirely or partially coated with gold or tin, as required. In use of the carrier tape, the inner leads of the finger leads are positioned over the electrodes on the IC devices and collectively bonded thereto so as to fasten a succession of IC devices to the carrier tape. The outer ends of the leads are then cut and connected to external leads. During this procedure, the sprocket holes 2 are used to position the tape relative to the IC devices for the purpose of the above-mentioned collective bonding and cutting steps.
Although photoetching techniques enable high accuracy in the manufacture of carrier tapes, special difficulties arise because the tape is very thin as compared with usual sheet materials for which the technique is used. Whereas with usual sheet material sprocket holes are formed first of all and these used to locate and feed the tape during subsequent processing, with thin metallic foil tape, the sprocket holes tend to deform, thus preventing accurate, collective bonding of IC devices. For this reason, pinch rollers have been used to feed the metallic foil tape. But this, too, is unsatisfactory, since, when the photoresist layer (which has been exposed to light through an elaborate mask) is pressed between the rollers, the latent patterns may be deformed, so that when these latent patterns are developed, the developed patterns for the sprocket holes and the finger leads are also deformed. To avoid this disadvantage it has been proposed in (for example) U.S. Pat. No. 4,227,983 to use a metal foil tape which is wider than the finished carrier tape, so that the tape may be driven by rollers gripping the margin of the tape. Where the metallic foil tape which has been coated with photoresist is subjected to continuous exposure to light, however, positioning of the tape by means of grip rollers and roller guides can lead to uneven spacing and inaccurate directional alignment between the successive sets of sprocket holes and finger leads to be formed in the tape.
The inventor has previously proposed a method of making a carrier tape in which perforations are formed at desired intervals in the tape margin portion by punching and that the position alignment for exposure is carried out using the perforations as disclosed in my copending U.S. patent application Ser. No. 535,862, filed on Sept. 26, 1983.
This proposed method enables more even spacing and greater directional accuracy as between successive sets of patterns than can be achieved using the grip roller system.
Various tools for the collective bonding of electrodes deposited on IC devices to inner leads have been proposed--see for example, Japanese Patent Publication No. 12790/78 and Provisional Publication No. 11664/79. However, because it is mechanically difficult to position the tape in the bonding tool for collective bonding using the sprocket holes immediately adjacent a particular cluster of finger leads, it is normal to use sprocket holes spaced some distance (upstream) from the cluster concerned for the purpose of aligning of the inner leads and electrodes.
When using a photomask 6, for example, having three successive sets of patterns for sprocket holes 4 and finger leads 5 as shown in FIG. 2A, a continuous carrier tape 1 is fed forward to present successive sections thereof to the mask to produce a continuing series of sets of sprocket holes 21, 22, etc., and finger lead patterns 31, 32, etc., as shown in FIG. 2B. Misalignment will not occur when the finger lead 31 in the exposure section (A) of the carrier tape 1 is intended to be aligned by means of the sprocket hole 22 or 23 (also in exposure section (A)) but, due to feed errors, misalignments may occur when it is required to position the finger leads 33 in the exposure section (A) using the sprocket hole 24 or 25 in the exposure section (B).
The reason for this is considered to be that the distance through which the tape is fed to bring successive sections thereof into the desired exposure position varies from one section to the next. That is, although alignment prior to exposure is carried out by sprocket holes which are provided in the margin (not shown) along each edge of the carrier tape 1 (as disclosed in the above-mentioned copending U.S. patent application), the accuracy of the punching operation to produce the sprocket holes in the margin determines the accuracy with which the tape is fed forward to bring the next section thereof beneath the mask. However, since the accuracy of the press is of the order of .+-.30 microns, it is almost impossible using tapes produced by conventional techniques, to accurately position finger leads relative to the IC devices. It is possible to decrease the frequency of bonding faults due to misalignment, by increasing the length, and hence also the number of pattern sets in the photomask but there is in practice a limit to the size of the section that can be exposed at one time. Consequently, bonding faults occur after constant intervals and cannot be avoided.